1. Field of the Invention
The present invention relates to an apparatus for discriminating a plurality of types of optical disks having different substrate thicknesses from each other and reproducing information therefrom, and more particularly, it relates to an apparatus for identifying each optical disk by a reproducing signal for the optical disk.
2. Description of the Background Art
There is provided an optical disk, such as a CD-ROM, having a thickness of about 1.2 mm for reading information with a semiconductor laser. In this type of optical disk, focus servo control and tracking servo control are performed on a pickup objective lens thereby irradiating a pit train of a signal recording surface with a laser beam and reproducing signals. In recent years, high densification progresses for recording dynamic images for a long time.
For example, an SD standard for recording information of about 5 Gbytes on a single side of an optical disk having the same diameter of 12 cm as a CD-ROM has been proposed. A single SD which is prepared by bonding two SDs each having a thickness of about 0.6 mm to each other can record information of about 10 Gbytes.
Japanese Patent Laying-Open No. 5-303766 (1993) proposes an apparatus for enabling reproduction of information from a high density optical disk provided with a thin substrate having a thickness of 0.6 mm and from a standard density optical disk provided with a substrate having a standard thickness of 1.2 mm with a single optical pickup.
This apparatus employs an objective lens having a numerical aperture of 0.6, which is designed to reproduce information from a high-density disk with a shorter-wavelength laser beam. An aperture for shielding an aberration corrector from an outer peripheral side of the laser beam thereby reducing the effective numerical aperture in case of reproducing information from an optical disk having a standard thickness and a standard density is inserted in a light source side of the objective lens.
In order to reproduce information from an SD as well as from a CD with a single pickup, it is important to identify the type of each optical disk which is set on a reproducing apparatus. Japanese Patent Laying-Open No. 6-259804 (1994) discloses a method related to this type of technique. This method is adapted to irradiate optical disks with light through an optical pickup, for discriminating the optical disks from each other by detecting differences between positions of reflected light from the optical disks.
In the technique disclosed in Japanese Patent Laying-Open No. 6-259804, each optical disk is irradiated with a beam emitted from a semiconductor laser unit which is stored in the optical pickup through an objective lens so that the optical disk is identified by the detected position of the reflected light. Thus, the operation for identifying each optical disk is required before starting an operation of reproducing information from the optical disk. Thus, the optical disk cannot be quickly identified. Further, a deflected optical disk cannot be correctly identified due to an influence of the deflection, since the position of the reflected light is detected.